5 SITE EVALUATION 5.1 Introduction Site selection is one of the most profound and vexing choices one faces in planning an observatory. The properties of the site impact the range of science that can be contemplated, the cost of construction and operations and the working environment for the observatory staff and visiting astronomers. The question of site is made all the more daunting because the choice is final and yet must be made on the basis of information that is incomplete at best. The GMT project is in the enviable position of having clear access to a developed site with a long history of excellent performance. The Las Campanas Observatory (LCO) in Chile has been known to be an outstanding site for more than 30 years. The quality of the seeing at LCO is as good, or better, than at any other developed site in Chile. There is negligible light pollution and little prospect of any in the future. The weather patterns have been stable over the past 30 years and there are reasons to believe that the cyclical variations present in the tropical zones do not reach the latitude of Las Campanas. It appears that there are no currently developed peaks in Chile that exceed LCO as an astronomical site. Las Campanas has been a field station for the Carnegie Institution of Washington since 1969. Carnegie has clear title to the land and a solid working relationship with the Chilean government and the major research universities. Other parties are able to construct and operate telescopes on Las Campanas under the umbrella of Carnegie’s agreement with the U. de Chile and several institutions have done so. Building the GMT at Las Campanas not only avoids potential legal and financial obstacles, it also reduces operation expenses by distributing infrastructure costs among other users of the site. The proximity of LCO to La Serena (the regional capital) provides a convenient logistical base and ensures a stable source of qualified employees who enjoy a high quality of life, a non-trivial consideration for an operation of the size and complexity envisioned for GMT. Several of the GMT partner institutions have an on-going commitment to Las Campanas via their participation in the Magellan twin 6.5m telescope consortium. This provides both strong scientific synergy between the two facilities and, potentially, simplifications and cost savings in operations. There are two known scientific drawbacks to the LCO site. The first relates to the fraction of clear nights. Las Campanas and the other mid-latitude sites (Cerro Tololo, Cerro Pachon, La Silla) have seasonal weather patterns that negatively impact the fraction of photometric nights in the Chilean winter. Sites to the north (e.g. Paranal) are affected less by winter weather, and so have a higher fraction of clear nights. However, the precise extent of this effect is unclear as the more northerly sites may suffer more from long term weather cycles. The counter argument is that locating GMT at a new site in the north of Chile, while improving the number of photometric nights, exposes the project to the risk of long-term variability of the seeing. This appears to be the case at Paranal, where the wind patterns have changed since the time of the VLT site survey. At this time, it is felt that the potential gains offered by a moderate elevation northern site are more than offset by the risks associated with selecting a new site on the basis of a short site survey. 5-1 The second drawback to LCO and sites of similar elevation is their relatively high levels of precipitable water vapor. Atmospheric water adversely impacts mid-IR astronomy through reduced transparency and increased background. There is no doubt that a high elevation (> 4000 m) site offers superior mid-IR characteristics compared to Las Campanas and similar sites However, constructing GMT at a 4000+ m site will increase the cost of construction and operations. At this time, the project has chosen to focus on development at LCO, while characterizing the site’s mid-IR performance. After two years of monitoring, it will be clearer if the site has sufficient performance in the mid-IR to meet the GMT science goals. After weighing both scientific and programmatic considerations, the GMT project, in concert with the GMT Board and Science Working Group, has designated Las Campanas as the baseline site and confined its site testing effort to characterizing sites within the LCO property. The aim is to identify the best peak within LCO in terms of seeing and wind speeds. The fraction of the time suitable for mid-IR observations is also being determined. The GMT project is in contact with the NOAO/NIO office regarding their site testing of Cerro Tolanchar at 4500 m as a potential high elevation alternative site for GMT. The Project believes that the most prudent approach is to maintain the project options with respect to a high site, while building on the significant advantages offered by unimpeded access to a first-rate mid-elevation site at LCO. If LCO is found to be unable to meet the science goals of GMT, then alternative sites will be considered based on data obtained from the NOAO/NIO site survey. This chapter describes the scientific context of the GMT site selection process and reviews the testing program underway at LCO. In Appendix 5-1, information regarding alternative ELT sites in Chile is provided. 5.2 CONSIDERATIONS FOR SITE SELECTION The choice of site will have a significant impact on the type of scientific investigations that can be carried out with the GMT. The critical aspects of the site vary from one science area, and observational technique, to another. While all science areas require clear weather and all benefit from photometric conditions, only some are sensitive to either water vapor or seeing. It is also true that near-IR and mid-IR science is largely insensitive to light pollution, but a site with significant light pollution, or a clear threat of such in the future, would be rejected in any case. Most premier astronomical sites are confined to coastal mountain ranges and islands. Viable sites at this time include the Hawaiian and Canary islands, the coastal ranges of Chile, Baja California, Namibia, and, possibly, continental sites in central Asia, although few of these have been characterized. Continental sites in North America suffer from light pollution and legal and environmental challenges that make development difficult. The seeing and mid-IR performance requirements of a site are somewhat subjective, as better seeing and lower water vapor are always desired, but they can be quantified somewhat with metrics based on the GMT science goals. The SWG has used the science requirements document to assess the impact of site selection with regard to PWV and seeing. For each science area, a sensitivity to water vapor can be assigned. Similarly, a sensitivity to seeing can be estimated, with the expectation that beyond 2 µm all sites will deliver good seeing and that beyond 5 µm the telescope should deliver diffraction-limited imaging nearly all of the time. It is worth noting 5-2 that while all visible wavelength science has a low sensitivity to PWV, not all mid-IR science areas are equally impacted by high PWV. Programs that are limited primarily by resolution rather than sensitivity may have only moderate sensitivity to PWV. Finally, to each science topic, a weighting factor must be assigned that is based on a combination of a qualitative assessment of its future scientific impact and the suitability of the GMT for this particular area. This metric provides guidance and balance in weighing the scientific promise of potential sites. 5.2.1 Hemisphere Choice While one might a priori consider sites anywhere on the globe, there are strong scientific motivations for a southern hemisphere location for the GMT. A location in the south makes the most sense in terms of both the science and the synergy with other facilities. Direct access to the Magellanic Clouds, the Galactic center, the bulk of the Milky Way star forming complexes and many of the best exoplanet targets, more than offsets the loss of access to M31 and other northern hemisphere-unique targets. Most of the current GMT partners are involved in the operation of the twin 6.5 m Magellan telescopes at Las Campanas and these telescopes can play a key role in supporting GMT science once it goes into operation. A site in the south would also offer unique opportunities for synergy with ALMA and, perhaps, the Square Kilometer Array, for which a southern site is likely, but not certain. 5.2.2 Impact of Cloud Cover The impact of cloud cover on the science capability of the facility is quite clear. The importance of the difference between the number of photometric nights and usable nights is not as obvious. It is unlikely that the GMT will be used for precise absolute photometry in the classical sense; all-sky 1% accuracy is not at the core of any of the science goals for this telescope. In the era of large sky surveys (e.g. the Sloan Digital Sky Survey, LSST) photometric zero-points and color terms can, in principle, be determined at, or near, any intermediate or high latitude location in the sky. Sites with low photometric fractions are, however, less productive than sites with high fractions of photometric and usable nights. Meeting the GMT science goals in a reasonable period of time requires that the fraction of photometric nights to be greater than 60% with a goal of greater than 70%.